docs: def _tab(element):
      """Extraction and caching of IAU2000 nutation coefficients
      """
  
      elements = {
          'x': 'tab5.2a.txt',
          'y': 'tab5.2b.txt',
          's': 'tab5.2d.txt'
      }
  
      if element.lower() not in elements.keys():
          raise ValueError('Unknown element \'%s\'' % element)
  
      filepath = Path(__file__).parent / "data" / elements[element.lower()]
  
      total = []
      with filepath.open() as fhd:
  
          for line in fhd.read().splitlines():
  
              line = line.strip()
  
              if line.startswith("#") or not line.strip():
                  continue
  
              if line.startswith('j = '):
                  result = []
                  total.append(result)
                  continue
  
              # The first field is only an index
              fields = line.split()[1:]
              fields[:2] = [float(x) for x in fields[:2]]
              fields[2:] = [int(x) for x in fields[2:]]
              result.append(fields)
  
      return total